Magnetic powder and preparation method thereof

ABSTRACT

Provided are a SmFeN magnetic powder which is superior not only in water resistance and corrosion resistance but also in hot water resistance, and a method of preparing the powder. The present invention relates to a method of preparing a magnetic powder, comprising: plasma-treating a gas; surface-treating a SmFeN magnetic powder with the plasma-treated gas; and forming a coat layer on the surface of the surface-treated SmFeN magnetic powder.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority to Japanese Patent Application No.2018-180239 filed on Sep. 26, 2018. The disclosure of Japanese PatentApplication No. 2018-180239 is hereby incorporated by reference in itsentirety.

BACKGROUND Technical Field

The present invention relates to a magnetic powder and a method ofpreparing the powder.

Description of Related Art

SmFeN-based bonded magnets are known as composite materials which may beused in motors for use in aqueous environments such as water pumps. Forexample, JP 2000-47802 A and JP 2005-286315 A disclose methods offorming a silica film on the surface of an oxidation-susceptibleSm-containing magnetic powder by a sol-gel process.

However, bonded magnets that are superior not only in water resistanceand corrosion resistance (oxidation resistance) but also in hot waterresistance are required particularly in in-vehicle applications. Merelyforming a silica film can improve water resistance and corrosionresistance, but further improvements in hot water resistance aredesired.

SUMMARY

The present invention aims to provide a SmFeN magnetic powder which issuperior not only in water resistance and corrosion resistance but alsoin hot water resistance, and a method of preparing the powder.

The present inventors conducted various studies on hot water resistanceof SmFeN magnetic powders. As a result, it has been found that not onlythe water resistance and corrosion resistance but also the hot waterresistance of SmFeN magnetic powders can be improved by surface-treatingthe powders with a plasma-treated gas, and then forming a coat layerthereon. This finding has led to the completion of the presentinvention.

Specifically, one aspect of the present invention relates to a method ofpreparing a magnetic powder, the method including: plasma-treating agas; surface-treating a SmFeN magnetic powder with the plasma-treatedgas; and forming a coat layer on a surface of the surface-treated SmFeNmagnetic powder.

Another aspect of the present invention relates to a magnetic powder,including: a SmFeN magnetic powder; a plasma-treated layer formed on asurface of the SmFeN magnetic powder; and a coat layer formed on asurface of the plasma-treated layer.

The method of preparing a magnetic powder of the present invention,which includes surface-treating a SmFeN magnetic powder with aplasma-treated gas, and then forming a coat layer thereon, providesmagnetic powders which are also superior in hot water resistance.

The magnetic powder of the present invention is a SmFeN magnetic powderwhich is superior not only in water resistance and corrosion resistancebut also in hot water resistance.

DETAILED DESCRIPTION

Embodiments of the present invention are described in detail below. Thefollowing embodiments, however, are intended as examples to embody thetechnical idea of the present invention and are not intended to limitthe scope of the present invention to the following embodiments. As usedherein, the term “step” encompasses not only an independent step butalso a step that may not be clearly distinguished from other steps, aslong as a desired object of the step is achieved.

A method of preparing a magnetic powder of a present embodiment mayinclude:

plasma-treating a gas;

surface-treating a SmFeN magnetic powder with the plasma-treated gas;and

forming a coat layer on the surface of the surface-treated SmFeNmagnetic powder.

The magnetic powder in the present embodiment includes a SmFeN magneticpowder. The SmFeN magnetic powder may be a nitride having a Th₂Zn₁₇-typecrystal structure and containing the rare earth metal Sm, iron (Fe), andnitrogen (N) as represented by the formula: Sm_(x)Fe_(100-x-y)N_(y)wherein the value “x” indicating the atomic percentage (%) of the rareearth metal Sm is in the range of 8.1 to 10 (at %); the value “y”indicating the atomic percentage (%) of N is in the range of 13.5 to13.9 (at %); and the balance is mainly Fe, more typically by Sm₂Fe₁₇N₃.SmFeN magnetic powders have stronger magnetic force than that of ferritemagnetic powders, and even relatively small quantities of SmFeN canprovide high magnetic force. Moreover, SmFeN magnetic powders have asmaller particle size than that of other rare earth magnetic powderssuch as NdFeB and SmCo and thus are suitable as filler for the matrixresin. Their another feature is that they are less likely to rust.

The magnetic powder may be a combination of a SmFeN magnetic powder witha NdFeB or SmCo rare earth magnetic powder or a ferrite magnetic powder.

The magnetic powder preferably has an average particle size of 10 μm orless, and the average particle size is more preferably 1 μm or greaterand 5 μm or less. With an average particle size of 10 μm or less,defects such as irregularities and cracks are less likely to occur onthe surface of the product, and the product has a better appearance.Moreover, a cost reduction can also be achieved. With an averageparticle size of more than 10 μm, defects such as irregularities andcracks may occur on the surface of the product, resulting in a poorappearance, while with an average particle size of less than 1 μm, thecost of the magnetic powder increases, and thus it is not preferred inview of cost reduction. Herein, the average particle size is defined asthe particle size corresponding to the 50th percentile by volume fromthe smallest particle size in a particle size distribution.

The plasma treatment of a gas may be performed by any method such asvacuum or atmospheric pressure plasma treatment. Among these,atmospheric pressure plasma treatment is preferred in that it eliminatesboth the need to take time for pressure reduction and the need forlarge-scale equipment such as vacuum chambers.

Examples of the gas for surface-treating the SmFeN magnetic powderinclude, but are not limited to, CF₄, argon, nitrogen, and air. Thesemay be used alone or in combination. Among these, CF₄ is preferred inthat it not only enhances adhesion of a silica film, but also will forma fluoride film, resulting in higher resistance to hot water.

It is sufficient that the molar ratio of the amount of the gas used inplasma treatment to the amount of the SmFeN magnetic powder should be0.005 or greater and 50 or less, preferably 0.05 or greater and 46 orless. When the molar ratio of the amount of the gas is less than 0.005,the plasma treatment tends to produce no effect, while when it exceeds50, the SmFeN magnetic powder tends to aggregate, resulting in reducedmagnetization.

Plasma exposure is performed to surface-treat the SmFeN magnetic powderwith the plasma-treated gas. The duration of plasma exposure is notparticularly limited, but is preferably one second or longer and 300seconds or shorter, particularly preferably 10 seconds or longer and 180seconds or shorter. When the duration of plasma exposure is shorter thanone second, the plasma treatment tends to produce no effect, while whenit exceeds 300 seconds, the SmFeN magnetic powder tends to aggregate,resulting in reduced magnetization.

The plasma-treated layer thus formed may have any thickness, preferablya thickness of 5 nm or greater and 300 nm or less. When the thickness ofthe plasma-treated layer is less than 5 nm, the plasma treatment tendsto produce no effect, while when it exceeds 300 nm, the SmFeN magneticpowder tends to aggregate, resulting in reduced magnetization.

Any coat layer may be formed on the surface of the surface-treated SmFeNmagnetic powder, and examples include metal oxides such as silica,phosphates, alumina, titania, and zinc oxide. Among these, silica ispreferred in that it may easily form a film and provide high resistanceto oxidation and water.

The formation of a coat layer may be carried out by any method, such asa sol-gel process, liquid phase deposition, thermal oxidation in anoxidation/diffusion furnace, deposition in a thermal CVD system, ordeposition in a sputtering system. According to the sol-gel process, acoat layer may be formed by mixing the magnetic powder with an alkylsilicate and then adjusting the mixture with an alkali or acid to analkaline or acidic pH to cause hydrolysis and condensation of the alkylsilicate.

The alkyl silicate may be a silicate represented by the followingformula:

Si_(n)O_((n−1))(OR)_((2n+2))

wherein R represents an alkyl group, and n represents an integer of 1 to10. The alkyl group may be, for example, methyl, ethyl, propyl, orbutyl. The alkyl group is preferably ethyl because of its inexpensivecost and simple handling without toxicity The value of n, which affectsthe molecular weight of the alkyl silicate, is preferably in the rangeof 1 to 10. When n is greater than 10, it is difficult to form a compactsilica layer. The amount of the alkyl silicate added is preferably onepart by mass or greater and 10 parts by mass or less, more preferablyone part by mass or greater and 5 parts by mass or less, depending on,for example, the type of alkyl silicate, and the type, shape, andparticle size of the magnetic powder.

The hydrolysis of the alkyl silicate may be carried out in the presenceof either a basic or acidic catalyst, preferably of a basic catalyst. Itis known that the mechanism of the hydrolysis and condensation reactionsof alkyl silicates in basic aqueous solutions is fundamentally differentfrom that in acidic aqueous solutions. In the case of the reactions inbasic aqueous solutions, the alkoxy groups of each alkyl silicate aresimultaneously replaced with hydroxyl groups. Thus, siloxane bonds growthree-dimensionally from Si to form a silica sol with a condensednetwork. The basic catalyst may be ammonia, a hydroxide of an alkalimetal or alkaline earth metal, or a metal hydroxide which is basic inwater. Preferred is ammonia which will fully volatilize without leavinga residue during the subsequent heating step. The hydrolysis andcondensation reactions of alkyl silicates in the presence of basiccatalysts can take place over a wide range of hydrogen-ionconcentrations (pH). In particular, in the present invention, it ispreferred to use a basic pH of 7.5 or higher, more preferably a pH inthe range of 8 to 13.

The amount of water added for hydrolysis is preferably 0.1 times orgreater and 3 times or less, more preferably 0.5 times or greater and 2times or less, relative to the theoretical amount required for thehydrolysis of the alkyl silicate. When the amount is less than 0.1times, the hydrolysis tends to be insufficient to form a compact silicathin film, while when it is more than 3 times, the rare earth magneticpowder tends to aggregate, resulting in reduced magnetization.

The coating of the surface of the magnetic powder with the treatingagent may be carried out in a high speed shear mixer under dryconditions. The coating may be accomplished by applying a silica soluniformly to the surface of the magnetic powder while vigorouslystirring and dispersing the magnetic powder by the shear force of themixer, without depending only on the wettability of the silica sol. Theoxidation resistance of the resulting silica film is greatly affected bywhether a silica sol is distributed as evenly and uniformly as possiblein this step.

For this reason, the alkyl silicate may previously be evenly distributedand coated on the magnetic powder, followed by mixing with a basicaqueous solution to achieve a more uniform coating. Mixing conditionsmay vary depending on the stirring speed, the capacity of the mixer, andthe size and shape of the blade, and the conditions should be selectedso that each system can be thoroughly and uniformly mixed.

The silica thin film with a three-dimensional network formed on thesurface of the rare earth magnetic powder particles may be heated tocause polycondensation of the remaining SiOH groups to stabilize thesilica thin film so that it can become a more rigid silica thin film.This treatment is preferably carried out at a temperature of 60 to 250°C., more preferably 100 to 250° C.

The thus prepared silica thin film, when coated at a thickness in therange of 0.01 to 0.5 μm, enhances oxidation resistance without impairingmagnetic properties.

A coupling agent may be used to improve wettability between the magneticpowder and the resin and magnet strength. The coupling agent may beselected depending on the type of resin.

A magnetic powder of a present embodiment includes a SmFeN magneticpowder, a plasma-treated layer formed on the surface of the SmFeNmagnetic powder, and a coat layer formed on the surface of theplasma-treated layer.

The SmFeN magnetic powder and the coat layer are as described above.

The plasma-treated layer preferably contains fluorine. When CF₄ is usedas a gas for plasma treatment, the resulting layer will containfluorine. The fluorine content is not particularly limited, but ispreferably 100 ppm or higher and 2000 ppm or lower, more preferably 400ppm or higher and 1500 ppm or lower. When the fluorine content is lowerthan 100 ppm, the plasma treatment tends to produce no effect, whilewhen it exceeds 2000 ppm, the SmFeN magnetic powder tends to aggregate,resulting in reduced magnetization.

The magnetic powder of the present embodiment may be mixed with a resinto prepare a bonded magnet composition.

Any resin may be used in the bonded magnet composition, and examplesinclude polypropylene, polyethylene, polyvinyl chloride, polyester,polyamide, polycarbonate, polyphenylene sulfide, and acrylic resins.Among these, polyamide resins are preferred, and polyamide 12 isparticularly preferred because it is a crystalline resin having arelatively low melting point and a low water absorption rate, and thusshows good moldability. Moreover, these resins may be used in admixtureas appropriate. The amount of the resin is not particularly limited, butis preferably 5 parts by mass or greater and 15 parts by mass or less,more preferably 6 parts by mass or greater and 12 parts by mass or less,per 100 parts by mass of the magnetic powder. An amount of less than 5parts by mass tends to provide poor moldability, while an amount greaterthan 15 parts by mass tends to result in lower magnetic properties dueto the reduced magnetic powder content.

The bonded magnet may contain components commonly incorporated in bondedmagnets, such as antioxidants, heavy metal deactivators, etc. When anantioxidant is incorporated, its amount is preferably 0.01 parts by massor greater and 5 parts by mass or less per 100 parts by mass of themagnetic powder. An amount of less than 0.01 parts by mass tends toproduce almost no antioxidant effect on the binder resin, while anamount greater than 5 parts by mass tends to reduce sliding propertiesof the melt and to significantly reduce mechanical strength of thebonded magnet.

Such a composite material of the present embodiment may be used inmagnets for water pumps for inverters or subradiators, sensors, smallmotors, and other similar applications.

EXAMPLES

Examples are described below. It should be noted that “%” is by massunless otherwise specified.

Examples 1 to 7

In each example, 100 g of a Sm₂Fe₁₇N₃ magnetic powder having an averageparticle size of 3 μm was subjected to plasma treatment in anatmospheric pressure plasma generator (microwave output power: 2 kW,exposure area: 100 mm in diameter) using the type of gas, flow rate ofthe gas, and duration of plasma exposure listed in Table 1.

Next, the plasma-treated Sm₂Fe₁₇N₃ magnetic powder was mixed with ethylsilicate (n=5) in a mixer, followed by adding aqueous ammonia adjustedto a pH of 12 and then mixing them for one minute. After completion ofthe mixing and dispersion, the resulting magnetic powder was taken fromthe mixer and heated under reduced pressure at 230° C. for 30 minutes toobtain a Sm₂Fe₁₇N₃ magnetic powder having a silica thin film formed onthe surface thereof.

Comparative Example 1

A Sm₂Fe₁₇N₃ magnetic powder having a silica thin film formed on thesurface thereof was prepared as in Example 1, except for omitting theplasma treatment.

Comparative Example 2

The magnetic powder of Comparative Example 2 was the plasma-treatedSm₂Fe₁₇N₃ magnetic powder obtained in Example 1. No silica thin film wasformed on the surface of the magnetic powder of Comparative Example 2.

Comparative Examples 3 and 4

Sm₂Fe₁₇N₃ magnetic powders having a silica thin film formed on thesurface thereof were prepared as in Example 1, except for changing theorder or timing of the plasma treatment step, as indicated in Table 1.

The Sm₂Fe₁₇N₃ magnetic powders prepared in the examples and comparativeexamples were measured for coercive force and fluorine (F) content asdescribed below. Table 1 shows the results.

Coercive Force

The magnetic powders prepared in the examples and comparative exampleswere left for 200 hours in a constant temperature and humidity chamberat 85° C. and 85% RH and then measured for coercive force using avibrating sample magnetometer (VSM) with a maximum field of 16 kOe.

F Content

The magnetic powders prepared in the examples and comparative exampleswere measured by inductively coupled plasma-atomic emission spectroscopy(ICP-AES).

TABLE 1 Atmospheric pressure plasma treatment Amount of Duration ofCoercive F Example Type of gas exposure Coat force content No. gas[L/min] [sec] Timing layer [0e] [ppm] Example 1 CF₄ + N₂ CF₄: 0.8 180Before coat layer Present 19300 1500 2 N₂: 3.2 10 formation 19400 460 330 19100 700 4 50 19400 800 5 N₂ 4 180 15400 0 6 Ar 4 180 18600 0 7 Air4 180 17000 0 Comparative 1 Plasma treatment: Not performed Present14700 0 Example 2 CF₄ + N₂ CF₄: 0.8 180 — Absent 13100 850 3 N₂: 3.2After coat layer Present 13700 1200 formation 4 Simultaneously withPresent 14000 2600 coat layer formation

Examples 1 to 7 exhibited improved heat resistance as their coerciveforces after the constant temperature and humidity test were higher thanthat of Comparative Example 1 prepared without plasma treatment.Comparative Example 3 in which atmospheric plasma treatment wasperformed after the formation of a silica coat layer, and ComparativeExample 4 in which atmospheric pressure plasma treatment was performedsimultaneously with the coat layer formation exhibited no plasmatreatment effect.

The magnetic powders and bonded magnet compositions of the presentinvention having high heat resistance are suitable for use inapplications such as water pumps.

What is claimed is:
 1. A method of preparing a magnetic powder, themethod comprising: plasma-treating a gas; surface-treating a SmFeNmagnetic powder with the plasma-treated gas; and forming a coat layer ona surface of the surface-treated SmFeN magnetic powder.
 2. The method ofpreparing a magnetic powder according to claim 1, wherein the gas isCF₄, argon, nitrogen, or air.
 3. The method of preparing a magneticpowder according to claim 2, wherein the gas is CF₄.
 4. A magneticpowder, comprising: a SmFeN magnetic powder; a plasma-treated layerformed on a surface of the SmFeN magnetic powder; and a coat layerformed on a surface of the plasma-treated layer.
 5. The magnetic powderaccording to claim 4, wherein the plasma-treated layer containsfluorine.
 6. The magnetic powder according to claim 5, wherein thefluorine is present in an amount of 100 ppm or higher and 2000 ppm orlower.